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Topic: Artficial star distance? (Read 378 times)

Looking to begin star testing my 80mm f5 and need some clarity for the distance of my artificial star. I'm guessing a 5mm eyepiece and approximately at least 60ft minimum according to my figures but isn't that too close? Thanks

<sub>50 feet or so is about right, if you got a long hallway the length of house can do it inside. My last house couldsetup scopeby front door aimed down hall, leave garage door open and put the hubble flashlight in dark on far wall, still needed an extension tube to get to focus. Stellarvue has a 4.75" for a reasonable price.</sub>

It does in fact depend on what you are trying to do with that artificial star. If you are just collimating, then such short lengths work. If you are evaluating the quality of the optics you'll get some spherical aberration at that distance.

There are two problems with the methods described here. One is air currents and the second one is wavefront overcorrection due to the source being too close. The simplest way to resolve both issues is to use a collimating telescope as show below.Mladen

An artifical star should be a light source, pin hole and a lens at a distance of the focal length.That way the output from the unit is delivering close to collimated light, to check the output you will have toi be able to move the lens in and out a little either side of the focal length.

Problem then is getting the pin hole and the lens centered.After that the whole lot has to sit on the optical axis of the scope.

I already have an artificial star. It's a Tech 2000 Nano-Star. The one thing I don't quite understand is that they tell you that you can bounce the light off of a ball bearing or similar to achieve a longer distance. Wouldn't that be the same as setting it up at the same distance as the ball bearing? Why not just use a dimmer light source? (Now I'm getting into a little bit of theory)

Simple principle - look at the reflection of a scene in a mirrored ball and compare to the angular diameter of the original scene. The convex reflector functions as a negative lens and shrinks everything including the size of the light source.

There are two problems with the methods described here. One is air currents and the second one is wavefront overcorrection due to the source being too close. The simplest way to resolve both issues is to use a collimating telescope as show below.

It should be mentionned that the collimating telescope needs have equal or lager aperture than the telescope to collimate.

An 80 mm f/5 has a 400 mm only. A long hallway in a school or university would do if it is longer than 20 f, as recommended in Harold Suiter's book.

Star test should ideally be done at higher magnification than 25X per inch of aperture, so a 5 mm eyepiece is not enough in my experience to really look at the diffraction pattern. When I collimated my ST-80, I used a 3.2 mm eyepiece. I would not use a barlow unless I knew it was near perfect! Too easy to blame the scope if the bad sfar test is caused by the barlow, as is the case with two of my cheaper barlows.

It should be mentionned that the collimating telescope needs have equal or lager aperture than the telescope to collimate.

In addition if the collimating scope has an obstruction you would need to go off axis to avoid the shadow. This places a limit on the aperture being tested.

For collimation only that can be relaxed. For a star test the incoming wavefront needs to be as flat as possible. This requires the pinhole to be placed at the infinity focal point rather accurately for generating the beam.

For a refractor of this type a long hallway is fine. Air currents will be seen, but longer distances are better as you dont want the drawtube extended too far as it might sag somewhat. Best to test closer to the intended design point, i.e. a longer distance to the art star.

A scope this size should have no problems with a real star even in average seeing.